Cathode for wet electrolyte capacitors

ABSTRACT

In an electrolytic capacitor, a layer of gold, platinum or gold platinum alloy constitutes the cathode, thereby permitting the capacitor to withstand reversals of polarity. Finely divided material such as carbon or platinum may be applied to the surface of the layer to increase the effective surface area of the cathode.

United States Patent Inventor James M. Booe Indianapolis, Ind.

Appl. No. 819,788

Filed Apr. 28, 1969 Patented Dec. 14, 1971 Assignee P. R. Mallory 8; Co.Inc.

Indianapolis, Ind.

CATHODE FOR WET ELECTROLYTE CAPACITORS 9 Claims, 1 Drawing Fig.

us. a

References Cited UNITED STATES PATENTS Burnham Hilton et al. WagnerSchroeder et al. Robinson et al,. ONair et al..

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Primary Examiner-James D. Kallam Altorneys- Richard H. Childress, RobertF. Meyer, Henry W.

Cummings and C. Carter Ells, .lr.

ABSTRACT: In an electrolytic capacitor, a layer of gold. platinum orgold platinum alloy constitutes the cathode, therehy permitting thecapacitor to withstand reversals of polarity. Finely divided materialsuch as carbon or platinum may be applied to the surface of the layer toincrease the effective surface area of the cathode.

Patented Dec. 14, 1971 3,628,103

I 20 vy VH1] H r INVENTQR ES M BOOE i I TTORN CATHODE FOR WETELECTROLYTE CAPACITORS ln some applications, it may be desired orrequired that capacitors used in circuits have the capability ofwithstanding reversed polan'ty. Some applicationsrequire the capacitorto withstand appreciable reversed polarity and for these applications anonpolar type of capacitor is employed. If the capacitor is of theelectrolytic type, then both electrodes are made of a film-forming metalsuch as tantalum, niobium, aluminum, etc. Such capacitors requireconsiderable space and are expensive. ln one conventional wetelectrolyte capacitor, a silver cathode is employed with a sulfuric acidelectrolyte and a tantalum or niobium anode. Reversal of potentialcauses silver to be dissolved in the electrolyte and, eventually, itwill be electrodeposited on the tantalum electrode after prolongedelectrification which is working 'as a cathode under this condition. Inthis capacitor system, only a few millivolts reversed polarity willcause appreciable reverse current. Upon reestablishing the normalpolarity, a high DC leakage current may result due to electrolyticdissolution of silver which has deposited on the tantalum electrode bythe reverse polarity. This may not permit the capacitor to immediatelyattain its normal voltage. This may continue until a substantial amountof silver is anodically dissolved from the tantalum electrode, afterwhich the capacitor may operate normally.

It is an object of the present invention to provide an inexpensivecapacitor of the wet electrolyte type having the capability ofwithstanding low values of reversed polarity.

It is another object of the present invention to provide a capacitorwhich will withstand reversals of polarity at elevated temperatures.

It is another object of the present-invention to provide a wetelectrolyte type capacitor which conserves space.

It is another object of the present invention to provide a wetelectrolyte capacitor which avoids the cathode being dissolved in theelectrolyte and deposited on the tantalum electrode during reversals ofpolarity.

It is another object of the present invention to provide a wetelectrolyte capacitor which gives low values of DC leakage duringreversals of polarity. 7

It is another object of the present invention to provide a wetelectrolyte capacitor which does not require'prolonged operation toobtain low values of DC leakage after there has been a reversal ofpolarity.

Other objects will be apparent from the following description anddrawing. Y I

The H0. is a schematic sectional view of the cathode and capacitor ofthe present invention.

In order to achieve the foregoing objects, an impervious layer of goldor platinum constitutes the inner surface of the cathode in contact withthe electrolyte. Additionally, certain alloys of gold and platinum, suchas Pt in Au up to about 25 wt. percent and gold in Pt up to about wt.percent may be used. Gold and platinumand these alloys are substantiallyunaffected when made anodic in many nonhalide electrolytes includingsulfuric acid. Although these metals may assume a slight oxidationduring operation as anode, they do not appreciably dissolve for examplein a sulfuric acid electrolyte with a tantalum electrode even after manythousands of hours of operation under reversed bias conditions to theextent of up to about [.5 volts and up to 2.0 volts and higher for shortperiods of time. I

If a reversed potential of much above this is applied, then appreciablecurrent will flow through the capacitor. Although this condition willnot attack the gold or platinum, oxygen gas may be liberated from-theelectrolyte at the electrolyte-Au/Pt interface and-hydrogen gas may beliberated from the electrolyte at the tantalum electrode. Theseconditions are deleterious to the capacitor.

Since gold and platinum are costly materials, it is highly uneconomicalto employ these metals as the entire cathode container, although thiscould be done if desired; gold, platinum and the gold-platinum alloysmentioned above may be formed into container cans by well-known methods.Preferably, however, the thinnest layer of Au or Pt or the Au- I Ptalloys mentioned above which are essentially pore free, is

applied to a can made of less expensive material. since the thicker theAu or Ft layer, the more costly the capacitor. For example, a layer ofabout 0.000l to about 0.005 inch, preferably 0.0005 to 0.003 inch thickmay be used as a liner inside a cathode container of some otheracceptable metal.

With respect to employing the thin films or layers of gold or platinum,or the above alloys of gold and platinum, these layers should beessentially free from imperfections such as pinholes which would permitthe electrolyte to come in contact with the supporting metal orstructural case or cladding metal of the capacitor and dissolve aportion of the case, which may then be electrodeposited at the tantalumelectrode during periods of reversed polarity. By essentially pore freeis meant that the layer must be sufficiently devoid of pores that uponreversals of polarity the ultimate leakage current from the anode to thegold, platinum or gold-platinum alloy is at most under 1 microampere'persquare inch cathode area at voltages up to L0 v.

Although virtually any metal can be used for the container which can beformed into a can and to which the liner can be afiixed, the preferredmetals are silver and copper and their alloys. Silver and copper andtheir alloys are preferred primarily because of their resistance tochemical attack including attack by electrolytes such as sulfuric acidshould there be imperfections in the gold or platinum lining, whichwould allow the electrolyte to permeate to the outer casing. In theabsence of oxygen or oxidizing agents, these metalsand alloys are notchemically attacked. The most preferred can materials are elementalcopper, elemental silver and alloys of Cu and alloys of silver forexample including brass. bronze and Ni-Cu alloys such as monel. Alsocans of low carbon steel, alloy steel or stainless steel or other strongmetals may be used where the application requires severe strength and/orrigidity.

As is known to those skilled in the art, there are various ways to applythe layer to the inner surface of the container and the invention is inno way to be limited by the particular method chosen. One methodinvolves electrodepositing the Au or Pt on a smooth mandrel, such aselectropolished stainless steel. This deposit is preferably burnished toclose any pores. The mandrel is then pressed into an outer'casingtoapply the layer. to the case. Another method involves masking one sideof a Ag or Cusheet and then .electroplatingthe layer on the unmaskedside. The plated sheet is then preferably rolled to close any pores,.then stamped and drawn into cathode-cases.

However, the preferred way is to form a bimetal of the can material forinstance, copper or silver, or their alloys with a thin layer of gold,platinum or gold/platinum alloys by roll bonding which ensures thelayer'will be essentially free of pores and, furthermore, ensures arelatively uniform thickness of the layer. For example, Ag and Au may beheated together at a temperature and time sufficient to form a diffusionbond, for example at 800 C. and then hot-rolled to enhance the bond andprovide initial reduction. One or more additional hot or cold rollingsteps with intermediate anneals if required may be carried out to obtaindesired gauge. The above operation should preferably be carried outunder clean, ambient conditions to avoid foreign substances or particlesbeing embedded into the surface to form pinholes or other imperfections.With these bimetals, the, cylindrical cathode cans or cases can be drawnor spun by standard methods well known in the art.

The gold, silver or Au-Ag alloy layer does not have sufficient surfacearea to enable good capacitor operation for many applications from thedynamic standpoint or when alternating current is flowing through thecapacitor. It is is desired to have the anode exhibit near its fullcapacitance, the cathode preferably has about I00 times the capacitanceof that of the anode or higher. in order to achieve this, it isgenerally necessary to increase the effective surface area of thecathode. One way of increasing the efi'ective surface is to employ anacceptable material having very high'surface area and having very highchemical and electrochemical resistance to the electrolyte. Thismaterial is applied to the surface of the gold, platinum, or Au/Ptalloy. For example, such materials include finely divided Pt, Au, Au/Ptalloys, carbon and mixtures of the foregoing.

Platinum for this use may be applied for example by electrodepositionfrom a platinizing solution such as chloroplatinic acid, H Ptcl or apaint composition of platinum black may be used. The carbon or graphitemay be applied as a paint having a suitable binder, for example, asdescribed in U.S. Pat. No. 3,243,316, to hold the particles in place andto maintain electrical contact with the gold, platinum or gold/platinumalloy surface. Other methods of application may be used within the scopeof the present invention, the foregoing being by way of example only.

The electrolyte to be used in the capacitor of the present inventionmust be one in which Au, Pt, or Au/Pt alloys do not dissolve anodicallyto an appreciable extent and which has a sufficiently high decompositionvoltage to withstand the reverse voltages and which has sufficientconductivity to operate effectively in normal operation. While sulfuricacid is the preferred electrolyte, it will be apparent to those skilledin the art that other electrolytes may also be used, for example,including nitric acid and phosphoric acid. For certain applications, itwould be permissible to employ the alkali metal salts of these acids aselectrolytes (Li, Na, K, Rb, Cs).

Capacitors utilizing the essentially pore-free Au, Pt or Au/Pt alloylayer on the cathode area, the ratio of reverse DC leakage current ofsilver cathode capacitors to capacitors having a gold coated cathode isusually at least 50,000. This ratio is reached within about 30 minutesof electrification at reverse voltage of about 0.8 volts. At lowerreverse voltages, this ratio is lower and at higher reverse voltages andlonger times it is generally higher, often going to as much as 100,000after about 1 hour.

While the DC leakage will generally increase with increasing cathodearea, the ratio obtained is substantially independent of the cathodearea.

The higher ratios are obtained on reversed voltage values of up to theorder of about 2' volts.

After a reversal of about 1 volt, the reverse DC leakage is nearly.always below microamps and generally below about 1.0 microamp 10 amps)for a cathode area of about 0.25 sq. inches after electrification and astable condition is achieved.

The capacitors may be operated on a continuous basis at reversedvoltages up to about 1.5 volts and can be occasio'nally subjected toreversed voltages of the order of about two volts and higher for shortperiods of time.

The capacitance which can be obtained with the capacitors of the presentinvention depends upon the capacitance of the anode and the capacitanceof the cathode according to the relation .l where C is the capacitanceof the capacitor, C, is the capacitance of the anode and C is thecapacitance of the cathode.

In general, C}- must be of the order of 100 times that of C for thecapacitor to develop near full capacitance of the anode. i

For example, the capacitors of the present invention will develop acapacitance of up to the order of 500 microfarads at 6 volts forward fora capacitor length of approximately elevensixteenths inch by seventhirty-seconds inch outside diameter. Higher and lower capacitancevalues can be obtained as desired by those skilled in the art such asabout 2 pt". in small caPaQ -tors to 2,000 If. depending on the size andvoltage.

The capacitors of the present invention may be operated at temperaturesas low as of the order of 70 C., depending upon the electrolyte, or ashigh as about +125C. However, at higher temperatures the reverse leakagecurrent increases above the room temperature values hereinbeforedisclosed.

An exemplary embodimentof the invention is shown in H6. 1. In thisdrawing a capacitor 10 is shown having a silver can or housing. 11. Thecan is coated with a layer of gold 12 according to one of the methodsdescribed hereinbefore. Additionally, a layer of finely divided materialsuch as platinum is applied to layer 12 at 13 to increase the effectivesurface area of the cathode. The anode is illustrated at 14 which isheld in place by spacer 15. A Ta lead 16 passes through a seal 17 which,for example, may be made of a suitable elastomeric material. It is heldin place within housing 11 by crimping as indicated at 18. 1

The seal 19 is illustrated as being very simple in construction in whichthe elastomeric material contains an opening for anode lead 16.Obviously, the particular seal construction may vary as desired by thoseskilled in the art to meet the requirements of particular applications.The Ta lead is welded to the anode at 19. An electrolyte, for example,sulfuric acid, fills the space 20 between the anode and the cathode. Theanode lead is conductively attached such as by welding or soldering to acircuit wire 21. The cathode lead 22 is attached to the can or housingby welding or soldering as indicated at 23.

it will be apparent to those skilled in the art that many other sealingarrangements may be used in conjunction with the cathode and capacitorof the present invention and still fall within the scope thereof.Obviously, other geometries and compositions may be used for the seal.

Likewise, differently shaped anodes may be used. Also, while tantalumhas been disclosed as the anode hereinbefore, other film-forming metals,particularly niobium, could also be used as the anode.

Furthermore, the invention is in no way to be limited to the particularshape of the can or housing illustrated, it being obvious to thoseskilled in the art that the wide variety of shapes and geometries may beused for the can or housing and still fall within the scope of thepresent invention.

1 claim:

1. A capacitor which will withstand reversals of polarity comprising afirst electrode normally the anode made of a film-forming metal selectedfrom tantalum and niobium;

a second electrode normally the cathode comprising a structural supportmember selected from the group consisting of copper, silver, silveralloys, copper alloys 'and steel, and an essentially pore-free layer ofmetal selected from the group consisting of gold, platinum andgoldplatinum alloys'adhering to and covering a surface of said supportmember; 1

an aqueous electrolyte selected from the group consisting of sulfuricacid, nitric 'acid, phosphoric acid, alkali metal slats of saidacids,'and mixtures thereof, in contact with said first electrode'andsaid pore-free layer of said second electrode, said electrolyte notappreciably dissolving metal from the second electrode pore-free layerand being substantially free of liberated oxygen at the interface withsaid layer upon intervals of reverse voltage;

2. A capacitor according to claim 1 in which substantially none of saidsecond electrode is chemically or anodically dissolved in saidelectrolyte.

3. A capacitor according to claim 1 in which a finely divided materialhaving chemical and electrochemical resistance to the electrolyte isapplied to said layer.

4. A capacitor according to claim 3 in which said material is selectedfrom the group consisting of finely divided platinum, finely dividedgold, finely divided gold-platinum alloys, finely divided carbon andmixtures thereof.

5. A capacitor according to claim 1 in which a nonhalide electrolyte isused.

6. A capacitor according to claim 1 in which said electrolyte issulfuric acid solution.

'7. A capacitor according to claim 1 in which said support and saidlayer are formed as a bimetal composite.

8. A capacitor according to claim 7 in which said support and said layerand readily coformed mechanically.

9. A capacitor according to claim 1 in which said layer is from about0.0001 to b 0.005 inches thick.

2. A capacitor according to claim 1 in which substantially none of saidsecond electrode is chemically or anodically dissolved in saidelectrolyte.
 3. A capacitor according to claim 1 in which a finelydivided material having chemical and electrochemical resistance to theelectrolyte is applied to said layer.
 4. A capacitor according to claim3 in which said material is selected from the group consisting of finelydivided platinum, finely divided gold, finely divided gold-platinumalloys, finely divided carbon and mixtures thereof.
 5. A capacitoraccording to claim 1 in which a nonhalide electrolyte is used.
 6. Acapacitor according to claim 1 in which said electrolyte is sulfuricacid solution.
 7. A capacitor according to claim 1 in which said supportand said layer are formed as a bimetal composite.
 8. A capacitoraccording to claim 7 in which said support and said layer are readilycoformed mechanically.
 9. A capacitor according to claim 1 in which saidlayer is from about 0.0001 to 0.005 inches thick.